1. Field of the Invention
The invention relates to silicon and silicon and aluminum diffused iron alloys and a process for manufacturing electric products such as, motor laminations and transformer cores from these alloys.
2. Description of the Prior Art
In making various electrical products, e.g. motor laminations and transformer cores, it is desirable to use thin gauge sheets of iron or an iron alloy as the product that contain diffused silicon or silicon and aluminum (hereinafter referred to as "iron alloy product"). Increasing the silicon or silicon and aluminum content of the iron alloy product, significantly improves the electromagnetic performance making these materials more attractive for electromagnetic components of electrical products.
First, it reduces eddy current losses and thus core losses associated with the material. This is accomplished by the increase in electrical resistivity associated with increased silicon or silicon and aluminum alloy content. This combination of properties is difficult to achieve with conventional methods for producing iron. Second, the magnetostriction of the iron is decreased, thereby reducing the mechanical and subsequently electrical losses. Thirdly, the coercive forces in the iron are reduced but these can also be reduced further by lowering the interstitial element content of the iron alloy product; e.g. carbon and nitrogen to remove pinning sites for magnetic domains. Lastly, the magnetic properties in the plane of a thin gauge iron alloy product sheet can be substantially non-oriented, textured or oriented. For example, in the manufacture of electric motor laminations, it is generally desirable to produce an iron sheet having a grain structure that is randomly oriented in the plane of the iron sheet with the &lt;111&gt; orientation removed from the plane of the sheet; as such, less energy is required to magnetize and demagnetize the material. However, in the manufacture of transformer cores, it is desirable to have an oriented grain boundary structure within the plane of the alloy product sheet which is oriented in the flux carrying direction.
The literature is replete with processes describing how to make silicon steel. These processes usually involve an iron based raw material having a silicon content of less than 3 wt. %. When the silicon content is increased further, the iron becomes more difficult to cold roll into thin gauge sheets; see U.S. Pat. No. 3,423,253. European Patent 0198084 and U.S. Pat. No. 3,224,909 both discuss a method for improving the magnetic characteristics of iron used in the manufacture of electrical products. The method involves casting the iron into an ingot, typically having greater than 3 wt. %, slabbing the ingot, and hot rolling to form a continuous band. Thereafter, the band is subjected to a plurality of cold rolling steps. After cold rolling, the iron or iron alloy containing (hereinafter referred to as "iron alloy") band is heated in a gaseous atmosphere containing a volatile silicon compound selected from, for example, silicon halides, silane, substituted silane, silicon tetraacetate and silicon tetrathiocyanate. Cold rolling the iron would also produce an iron alloy having an undesirable equiaxial grain structure. In addition to this, the interstitial content of the iron alloy could not be reduced to the desired low levels without further processing, unless a high cost, special steel making practice was employed.
There has been little success in economically manufacturing iron containing 6.5 wt. % Si and greater where conventional processing techniques can be employed, e.g. cold rolling, since iron alloy having these elevated silicon levels embrittles easily and is not amenable to cold rolling. A developmental procedure that evolved required a rapid solidification of the iron and forming iron sheets directly from an iron melt. See U.S. Pat. No. 4,142,571. However, this process is expensive and therefore impractical for manufacturing large quantities of iron sheets for making electrical products.
The earliest description of using chemical vapor deposition for increasing the silicon content of silicon steel sheet is in U.S. Pat. Nos. 3,224,909 and 3,423,253. Further improvements of these chemical vapor deposition methods for fabricating steel of high silicon content were described by K. Nakaoka, et al., European Patent 85904865.4 and the properties of silicon-steel sheet of approximately 6.5 wt. % Si have been described by Takada, Abe, Masuda and Inagaki, J. Appl. Phys. 64 (10), pp. 5367-5369 (1988). Nakaoka describes siliconizing a steel sheet, containing 3 or more wt. % Si, in a flowing gas stream to increase the silicon content further to about 6.5 wt. %. The 3 wt. % Si sheet was heated in an atmosphere containing silicon tetrachloride in concentrations up to 50% for times up to 50 minutes, at a temperature between about 1100.degree. C. and about 1200.degree. C., at a controlled heating and cooling rate, to obtain high permeability silicon-steel sheet of about 6.5 wt. % Si, without internal flaws. After the siliconizing step, the silicon sheet was then annealed for homogenization at a temperature of about 1200.degree. C. for about two hours.
U.S. Pat. No. 4,904,500 discloses a pack diffusion technique where iron or iron alloy is placed in a retort, or chamber, and diffused with silicon by catalyzed oxide reduction. However, unlike the diffusion process described in U.S. Pat. No. 3,224,909 mentioned above, the diffusant species in pack diffusion is not introduced directly as a gaseous species, but is derived as product of the concurrent chemical reaction of reagents inside the retort. To effectively diffuse silicon into iron and preclude the outward diffusion of iron therefrom, the activity of the gaseous stream species or the diffusant species must be maintained at a sufficiently low level to allow the deposited silicon to be adsorbed by the substrate as soon as it is deposited on the surface.
Although the techniques described above can produce silicon steel sheet with silicon contents in the range 3 wt. % Si, they involve numerous manufacturing steps, and in particular use an expensive silicon steel sheet of greater than about 3 wt. % Si as the starting material. No methods have been described which allow the use of low silicon content material, including low carbon steel as the starting material, which offers considerable cost savings on materials. In addition, synthesis of silicon steel sheet in near net shape for cost improvements is not described. The present invention offers such advantages.
An object of this invention is to provide an iron alloy product having silicon or silicon and aluminum diffused therein, produced from an iron feedstock containing less than about 2.5 wt. % silicon or aluminum or a combination of silicon and aluminum, wherein the alloy product (a) is in the form of thin gauge iron sheets; (b) has a low interstitial content (c) has a columnar grain boundary structure within the plane of a sheet of the iron alloy product, wherein the magnetic properties within a plane of the sheet of the iron alloy product is substantially non-oriented, textured or grain-oriented; (d) has improved magnetic characteristics; (e) has reduced core loss; and (f) has reduced magnetostriction.
Another object of the invention is to provide methods for making an iron alloy product having silicon or silicon and aluminum diffused therein for the manufacture of electric motor laminations and transformer cores with reduced core loss and reduced magnetostriction.
Another object of the invention is to provide a method for manufacturing electric products, such as motor laminations and transformer cores, that reduces the amount of wasted metal used in the manufacturing process.
Other objects of the invention will become apparent to those skilled in the art upon reading the following description, to be taken in conjunction with the specific examples provided herein for illustrative purposes.